Emit a per-node clickhouse_node tag across all node-scoped ClickHouse checks

[sangeetashivaji]

What does this PR do?

Adds a uniform, additive per-node tag (clickhouse_node, value = ClickHouse hostName()) to every ClickHouse check that carries node-specific data, so metrics/samples/etc. can be sliced (and billed) per node. database_instance stays cluster-level, so existing cluster grouping/filtering is unaffected.

Changes by area:

• utils: new node_tag() helper (reuses the existing CLUSTER_NODE_TAG = 'clickhouse_node').
• Standard metrics (get_queries): system.replicas and system.dictionaries are now read via clusterAllReplicas and tagged per node in single-endpoint mode. system.parts is intentionally left plain — its GROUP BY is incompatible with the cluster_aware_query rewrite; per-node parts data comes from the DBM parts/merges check.
• Query metrics (statements.py): rows are no longer merged across nodes (_merge_rows_across_nodes removed). Rows are grouped by node and emitted as one payload per node, each carrying clickhouse_node. FQT events are node-tagged too.
• Query completions / errors / samples: payloads are split per node, each tagged clickhouse_node. ACTIVE_QUERIES_QUERY / ACTIVE_CONNECTIONS_QUERY now select hostName().
• parts_and_merges / table_metrics (view refreshes): clickhouse_node added alongside the existing server_node: / host: tags (kept for backward compatibility).

Motivation

ClickHouse Cloud billing is moving to per-node. Today the resource primary key is database_instance, which is cluster-level, and most check data is aggregated across nodes before it leaves the Agent — so there is no reliable way to attribute metrics to a node. This makes every node-scoped check emit a consistent per-node tag.

Breaking behavior: query-metrics per-query series change from cluster-summed to per-node; cluster totals must be re-aggregated across clickhouse_node. Acceptable while the feature is in preview.

Open dependency: the per-node approach emits multiple database_monitoring_query_metrics / query_activity payloads for the same (database_instance, timestamp). This assumes the DBM intake does not dedupe on that key — needs confirmation with the dbm-metrics-processor team before merge.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[github-actions]

⚠️ Major version bump
The changelog type changed or removed was used in this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the fixed or added type instead.

[dd-octo-sts]

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• Overall Coverage: 94.27% (+5.96%)

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